1. Field of the Invention
This invention relates to field emission displays, and more particularly to the removal of contaminants from the phosphor screens of such displays.
2. State of the Art
Cathode ray tube (CRT) displays, such as those commonly used in desktop computer screens, function as a result of a scanning electron beam from an electron gun impinging on phosphors on a relatively distant screen. The electrons increase the energy level of the phosphors. When the phosphors return to their normal energy level, they release the energy from the electrons as photons of light. The photons are transmitted through the glass screen of the display to the viewer.
Phosphors are known to decline in efficiency with use. There is an initial dramatic decrease in efficiency, followed by a slower, more gradual degradation over time. After the initial decrease in phosphor efficiency, the phosphors luminesce in a more uniform manner.
Phosphor coated screens are typically treated to bring about this initial aging, also known as xe2x80x9cbrowning,xe2x80x9d prior to the sale of the display. Electron beam xe2x80x9cscrubbingxe2x80x9d is one approach currently used in the manufacture of cathode ray tubes (CRT) to burn-in or age the phosphors. This process has been accomplished with the use of the electron (cathode ray) gun after the display has been assembled. Hence, the process is performed at duty cycles similar to those used for viewing purposes, as illustrated in FIG. 1. In other words, the normal operation of the CRT serves to cleanse the screen.
Flat panel displays have become increasingly important in appliances requiring lightweight portable screens. Currently, such screens use electroluminescent or liquid crystal technology. A promising new technology is the use of a matrix-addressable array of field emission microtips to excite phosphors on a screen.
Similar to cathode ray tubes (CRT), field emission displays (FED) are comprised of an anode and a cathode. The anode comprises a phosphor coated glass plate. The phosphor luminesces when it is bombarded by electrons. However, unlike a CRT, the field emission device has a cold cathode which is comprised of arrays of micro-miniature field emitters.
The xe2x80x9cscrubbingxe2x80x9d or cleansing of the phosphor screen to remove contaminants would also be fabrication of field emission displays. However, most of the known xe2x80x9cscrubbingxe2x80x9d methods are inappropriate for such displays.
One reason current xe2x80x9cscrubbingxe2x80x9d techniques are unworkable in a field emission display (FED) is that the micro-miniature cathode emitter tips are much more sensitive to contamination than the large cathode ray gun of CRT. If the contaminants dissociated during the scrubbing process react with the cathode emitter tips, the emission, and resulting display performance, will be degraded. Hence, the standard CRT method of electron beam scrubbing is not a practical approach for use in field emission displays.
The inventors of the present invention have determined that the degradation in performance of the field emitters is a result of the oxygen dissociated from the phosphors of the display screen. If the emitter tips are fabricated from a silicon-based material, the presence of oxygen may result in the formation of silicon dioxide. An oxidation layer functions as an insulator, thereby inhibiting electron emission. Hence, in a field emission display, the cleansing process must remove oxygen and as well as other contaminants from the display screen.
Another difference between CRTs and FEDs is that field emission displays employ low voltage phosphors compared to those used in CRT displays. Low voltage phosphors characteristically luminesce at voltages less than 5000 V. The difference in the type and quality of the phosphors used in field emitter displays also necessitates the development of a new cleansing process for the screens used therein.
Despite the apparent benefits that could be achieved from xe2x80x9cscrubbingxe2x80x9d the phosphor screens, in the present manufacture of field emitter displays, the screens are not scrubbed appropriately, if they are scrubbed at all. Consequently, it is necessary to accept a certain amount of degradation in the emitter performance. Therefore, there is a need in the industry for a screen cleansing method that will be effective when used for field emission displays.
One advantage of the present invention is that after the screen is properly scrubbed and the display is assembled, the display emitters will not experience a degraded performance as a result of contamination. This is a significant improvement, as degradation limits the lifetime and quality of the display.
In an example process in accordance with the present invention, the phosphor screen of the field emission display is bombarded by electrons at very high current densities (for example 1.0 mA/cm2) and at low energies (for example those less than 1000 V) in order to remove oxygen contamination from the phosphors and/or phosphor binder material, prior to sealing the screen on the display. The removal of the oxygen prevents the degradation of the field emitters in the display during operation.